A magneto-optical disk memory has been put into practical use as a rewritable optical disk memory. However, in order to record information again in the already-recorded portion, it is necessary to record it after erasing previously recorded information. Therefore, it has a disadvantage in that it takes a long time to re-record.
In order to solve this problem, magnetic modulation overwriting is used as a method to perform overwriting, that is, recording without erasing already-recorded information before re-recording.
However, this method performs overwriting by modulating the size of the magnetic field to be applied. Therefore, it has a disadvantage in that a device for generating a magnetic field has to be arranged in the vicinity of the magneto-optical disk in order to obtain a magnetic field of a sufficient size. Moreover, it also has a disadvantage in that recording takes much time because it is not possible to modulate the magnetic field at a high speed.
Accordingly, in order to solve the above-described problems, Japanese Laid-Open Patent Application 175948/1987 (Tokukaisho 62-175948) discloses an optical modulation overwriting method. This method uses a magneto-optical recording medium having a double-layered structure composed of a recording layer and a recording auxiliary layer each of which is a film having a perpendicular magnetization, and modulates only the laser power of the light beam, thereby enabling overwriting.
This method, disclosed in Japanese Laid-Open Patent Application 175948/1987, enables optical modulation overwriting. However, since the magnetization direction of the recording auxiliary layer is changed when overwriting, it is necessary for each recording process to initialize the magnetization direction of the recording auxiliary layer before overwriting. Therefore, besides a device for generating a magnetic field for recording, a device for generating an initializing magnetic field is necessary. As a result, increased size and manufacturing costs of the apparatus are problems.
Moreover, another problem of conventional magneto-optical disk memories is as follows: when the diameter and the interval of the recorded bit is small compared with the diameter of the light beam of the focused semiconductor laser, the reproducing property deteriorates. This is because the focused laser beam covers adjacent recorded bits therein, thereby making it impossible to reproduce each recorded bit separately.
In order to solve such a problem, Japanese Laid-Open Patent Application 81717/1993 (Tokukaihei 5-81717) discloses a technique using a magneto-optical recording medium of a double-layered structure. Namely, this medium has both a readout layer, which is a film showing an in-plane magnetization at room temperature and which becomes a film showing a perpendicular magnetization according to rises in temperature, and a recording layer, which is a film showing a perpendicular magnetization. This disclosure proposes an optical modulation overwriting method which reproduces each recorded bit separately even if the recorded bit is smaller than the diameter of the laser beam, and which does not need an initializing magnetic field.
However, in the technique proposed in Japanese Laid-Open Patent Application 81717/1993, since the recording layer and the readout layer are directly in contact with each other, a magnetic exchange-coupling force therebetween is strongly exerted. Therefore, since the technique cannot easily control the magnetization direction of the readout layer by using the recording magnetic field, it has a problem in that the optical modulation overwriting property is unstable.